This invention relates in general to molds for casting vehicle wheels and in particular to a vehicle wheel mold having a screenless gate.
Vehicle wheels have a circular wheel disc attached to an annular wheel rim. The wheel disc includes a central wheel hub having a pilot hole and plurality of wheel mounting holes formed therethrough. A plurality of equally circumferentially spaced spokes typically support the wheel hub within the wheel rim. The wheel rim is adapted to support a pneumatic tire.
In the past, vehicle wheels typically have been formed entirely from steel. However, wheels formed from light weight metals, such as aluminum, magnesium and titanium or alloys thereof, are becoming increasingly popular. In addition to weighing less than conventional all-steel wheels, such light weight wheels can be manufactured having a pleasing esthetic shape. Weight savings also can be achieved by attaching a wheel disc formed from a light weight metal alloy to a steel wheel rim.
Light weight wheels are typically formed by forging or casting operations. During a forging operation, a heated billet of the light weight metal alloy is squeezed by very high pressure between successive sets of dies until the final shape of the wheel is formed. During a casting operation, molten metal is inserted into a cavity formed in a multi-piece wheel mold. After the metal cools sufficiently to solidify, the mold is opened and a rough wheel casting is removed. The wheel casting is then machined to a final shape. Machining can include turning the outside and inside surfaces of the wheel rim, facing the inboard and outboard wheel disc surfaces and drilling the center pilot hole and the mounting holes through the wheel hub.
Conventional casting operations include numerous processes, such as die casting, low pressure injection casting and gravity casting. All the conventional casting operations typically utilize a wheel mold formed from a number of segments. The wheel mold defines a mold cavity which includes a rim cavity for casting the wheel rim and a disc cavity for casting the wheel disc.
For high volume production of castings, such as vehicle wheels, highly automated gravity casting processes are frequently used that typically use a casting machine having a plurality of molds mounted upon a moving structure, such as a rotatable carousel. Each mold is indexed past a refractory furnace containing a pool of molten metal. A charge of molten metal is poured into a gate formed in the mold which communicates with the mold cavity. Gravity causes the metal to flow from the gate into the mold cavity, filling the rim and disc cavities. The mold and the molten metal cool as the casting machine indexes the other molds to the refractory furnace for charging with molten metal. After a sufficient cooling time has elapsed, the mold is opened and the wheel casting removed for machining to a final shape. The mold is then closed and again indexed to the refractory furnace to be refilled with molten metal.
Referring now to the drawings, a sectional view of a typical known gravity casting wheel mold 10 is shown in FIG. 1. The mold 10 is formed from a high temperature resistant metal, such as a steel alloy. The mold 10 includes a base segment 12 which can include a plurality of subsegments. The mold 10 further includes a pair of movable side segments, one of which is shown in FIG. 1 and labeled 18. Each of the side segments is supported by the base segment 12 and can include a plurality of subsegments. The side segments can be extended to a closed position or retracted to an open position by a conventional mechanism which, for clarity, is not shown in FIG. 1. The side segments carry a pair of gate members, one of which is shown in FIG. 1 and labeled 20. The gate member 20 extends from the right side of the side segment 18 in FIG. 1. The gate members co-operate to form a gate 21 that receives molten metal for casting the wheel. The gate 21 includes a tappered inlet chamber 22 into which the molten metal is poured. The inlet chamber 22 communicates through a gate passageway 25 with an intermediate chamber 26 formed within the gate member 20. A narrow axial opening 27 is formed through the inner wall of the side member 18, the purpose for which will be explained below.
The mold 10 also includes a filter, or screen, 28, which is formed from a porous material, which is typically a ceramic, such as, for example, alumina foam, zirconia, silicon carbide or mica, is disposed across the base of the inlet chamber 22. Alternately, the filter can comprise a fiberglass screen (not shown). As shown in FIG. 1, the filter 28 is received in the bottom of the inlet chamber 21 and supported by a shoulder 29 formed therein.
The side segments receive an axially movable top segment 30. The top segment 30 can be extended to a closed position and retracted to an open position by a conventional mechanism which, for clarity, is not shown in FIG. 1. Similar to the other segments, the top segment 30 can include a plurality of subsegments. A ball riser segment 32 having an inverted cup shape is mounted in the center of the top segment 30. The ball riser segment 32 defines an interior chamber that is referred to as a ball riser cavity 34 in the following. A vent opening 36 is formed through the top of the ball riser segment 32.
When the top and side segments are extended to their closed positions, the mold 10 is closed and the top segment 30 cooperates with the base segment 12 and the side segments to define a mold cavity 40 for casting a vehicle wheel. The mold cavity 40, as shown in FIG. 1, includes a generally circular disc cavity 42 for casting the wheel disc and an annular rim cavity 44 for casting the wheel rim. The disc cavity 42 communicates with the ball riser cavity 34 while the rim cavity 44 terminates in an annular rim riser cavity 46. As described above, the axial opening 27 in the side segment 18 provides communication between the gate intermediate chamber 26 and the mold cavity 40.
The operation of the apparatus 10 will now be described. The side and top segments are moved to their extended positions to close the mold 10. Molten metal is poured into the gate 21. Gravity causes the molten metal to flow through the filter, or screen, 28 and the gate passageway 25 and into the intermediate chamber 26. The filter 28 removes oxides and other impurities from the molten metal. The filter 28 also reduces turbulence in the molten metal as the mold cavity 40 is filled, reducing oxidation of the molten metal. From the intermediate chamber 26, molten metal flows through the axial opening 27 and into the mold cavity 40. The molten metal flows across the disc cavity 42 and into the ball riser cavity 34. Similarly, molten metal fills the rim cavity 46 and enters the rim riser cavity 46. Pouring continues until the gate inlet chamber 22 is filled with molten metal. Contraction occurs as the molten metal solidifies, and molten metal flows from the rim and ball riser cavities 34 and 46 to fill any voids caused by the shrinkage. After the casting has cooled sufficiently, the top and side segments are retracted from the base segment 12, allowing removal of the casting.
During the casting operation, the filter 28 solidifies with the metal of the sprue formed in the gate 22. After each casting operation, the filter 28 removed with the wheel casting and discarded with when the sprue is cut from the casting. Accordingly, it is necessary to insert a new filter 28 into the gate before using the mold to cast another wheel. The replacement of the filter 28 is a time consuming operation and thus adds to the cost of manufacturing the wheel Additionally, the cost of the replacement screens further increases the total wheel manufacturing costs. Therefore, it would be desirable to provide an alternate reusable device in place of the screen.